An industrial robot is an example of a robot whose position is determined by varying its joint angle. Examples of a welding operation using a welding robot, which is a type of industrial robot, include an operation called touch sensing that senses the position of a workpiece to be welded. Touch sensing is a sensing operation which involves moving a welding robot while applying a voltage to a welding torch, and detecting a position at which a welding wire of the welding torch contacts the workpiece (i.e., a position at which electrical conduction between the workpiece and the welding wire is detected) as a workpiece position.
The touch sensing normally detects the position of the workpiece on the basis of the position of the robot (i.e., the motor angle for each joint of the robot) when the contact with the workpiece is detected. However, there is a delay between detection of the contact with the workpiece and acquisition of information about the position of the robot. If the welding wire is moved at a high speed and brought into contact with the workpiece, the welding wire is significantly moved even during the delay time. As a result, a position which is displaced from the actual workpiece position is detected as the position of the workpiece.
Therefore, when the welding wire is brought close to the workpiece, the welding robot is operated at a low speed to allow the welding wire to move at a low speed. However, operating the welding robot at a low speed is disadvantageous in that it takes time to perform the sensing.
As solutions to the problems described above, Patent Literature (PTL) 1 discloses a workpiece detecting method for an automatic welding apparatus, and PTL 2 discloses a wire touch sensing method for a welding robot.
The workpiece detecting method disclosed in PTL 1 is a workpiece detecting method for an automatic welding apparatus. The automatic welding apparatus includes conduction detecting means for selectively applying a welding voltage and a sensing voltage to a consumable electrode type welding torch, and detecting, during application of the sensing voltage, a state of electrical conduction between a consumable electrode protruding from the welding torch and a workpiece. The workpiece detecting method includes bringing the welding torch close to the workpiece at a high speed while applying the sensing voltage to the welding torch, stopping the motion of the welding torch in response to a conduction detection output from the conduction detecting means, detaching the welding torch at a low speed in accordance with the conduction detection output, detecting by the conduction detecting means the detachment of the consumable electrode from the workpiece, and using a detachment detection output from the conduction detecting means as a control signal for the automatic welding apparatus.
The wire touch sensing method disclosed in PTL 2 includes moving a welding wire while applying a voltage thereto, moving the wire backward at a low speed upon detection of a signal indicating a short circuit in the wire, and determining the position of detecting a short-circuit cancellation signal as a position at which an object to be welded is actually located.
In the techniques disclosed in these patent literatures, the welding wire is brought into contact with the workpiece while a voltage is applied to the welding torch, and the robot is stopped after detection of contact with the workpiece. Then, the welding wire is moved at a low speed to be detached from the workpiece, and the position of the workpiece is detected on the basis of the robot position when the welding wire and the workpiece are brought out of conduction.
In this method, after the welding wire is brought close to the workpiece at a high speed and then temporarily stopped, the workpiece position is detected during the subsequent detachment motion. Thus, the workpiece position can be accurately detected in a short sensing time.
As a technique for stopping the welding wire that has been brought close to the workpiece at a high speed, PTL 3 discloses an acceleration/deceleration method for an industrial machine.
The acceleration/deceleration method disclosed in PTL 3 is an industrial machine acceleration/deceleration method for controlling acceleration and deceleration of moving means in an industrial machine. The industrial machine includes a machine base whose relationship with an installation location is approximated to a first spring vibration system; and a control object having the moving means whose relationship with the machine base is approximated to a second spring vibration system, the moving means being configured to move over the machine base in response to a force generated by motion of an actuator secured onto the machine base. To accelerate and decelerate the moving means with a constant acceleration, a velocity command is generated which makes the acceleration time and the deceleration time equal to a period which is an integral multiple of the natural vibration period of the machine base determined on the basis of the sum of the masses of the machine base and the actuator and the spring constant of the first spring vibration system, and a motion command that causes the moving means to move on the basis of the velocity command is output to the actuator.
By generating a deceleration path using the acceleration/deceleration method based on the natural vibration period, the industrial machine can be stopped without vibration.